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Abstract:

A system and method for handoff are provided. A mobile station performs a
make-before-break handoff of a control channel between a serving and
target base station and a break-before-make handoff of a traffic channel
between the serving and target base stations. The traffic channel handoff
is performed after the control channel handoff has completed.

Claims:

1. A method for a mobile station handoff from a serving base station to a
target base station, the method comprising the acts of:performing, by the
mobile station, a make-before-break handoff of a control channel between
the serving and target base stations; andperforming, by the mobile
station, a break-before-make handoff of a traffic channel between the
serving and target base stations.

2. The method of claim 1, wherein the mobile station performs the handoff
of traffic channels only after the mobile station performs the act
of:indicating completion of the signaling channel handoff.

3. The method of claim 2, wherein the completion of the control channel
handoff is performed over a handoff switching channel.

4. The method of claim 1, wherein the mobile station:receives data from
the serving base station during a first and second time period;transmits
control information to the target base station during a time period
between the first and second time periods; andreceives data from the
target base station during a time period subsequent to the second time
period, wherein the mobile station receives data from only one base
station during any particular time period.

5. The method of claim 4, further comprising the act of:receiving, from
the serving base station, a message instructing the mobile station to
attempt to handoff to the target base station.

6. The method of claim 5, wherein the message identifies a reserved
ranging channel of the target base station and a connection
identification to be used for communicating with the target base station.

7. The method of claim 6, wherein the message also identifies the time
period between the first and second time periods.

8. The method of claim 4, wherein the mobile station receives a traffic
channel allocation from the target base station during the time period
subsequent to the second time period.

9. The method of claim 1, whereinthe mobile station is informed of a first
set of time periods for communicating with the target base station over
the control channel,information is scheduled for transmission to the
mobile station during time periods other than the first set of time
periods, andthe scheduled information is received by the mobile station
over a first traffic channel during periods of time other than the first
set of time periods.

10. The method of claim 9, wherein the information for transmission to the
mobile station includes unicast and multicast data.

11. The method of claim 9, wherein the first set of time periods are every
other n frames.

12. The method of claim 9, wherein the handoff is an intra-frequency
handoff.

13. The method of claim 9, wherein the handoff is an inter-frequency
handoff.

14. The method of claim 1, wherein the control channel is a first control
channel and the traffic channel is a first traffic channel, and wherein
the mobile station:communicates with the serving base station over the
first traffic channel and the first control channel during a first time
period;communicates with the target base station over a second control
channel during a second time period;communicates with the serving base
station over the first traffic and control channels during a third time
period, which is subsequent to the second time period; andcommunicates
with the target base station over the second control channel and a second
traffic channel during a fourth time period.

15. The method of claim 1, wherein the mobile station:identifies a base
station as the target base station;exchanges control channel messages
with the target base station; andcommunicates over the traffic channel
with the serving base station while exchanging the control channel
messages with the target base station.

16. The method of claim 15, further comprising the acts of:receiving a
feedback channel allocation from the serving base station;
andtransmitting an identification of the target base station to the
serving base station over the feedback channel.

17. The method of claim 16, further comprising the act of:receiving, from
the serving base station, a signaling channel allocation for the target
base station, wherein at least one of the control channel messages is a
ranging message which is transmitted over the allocated signaling
channel.

18. The method of claim 15, further comprising the acts of:receiving a
connection identification from the target base station; andcommunicating
over an allocated traffic channel with the target base station.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]The present application is a divisional application of U.S. patent
application Ser. No. 11/199,178, filed on Aug. 9, 2005, which claims
priority under 35 U.S.C. §119 to U.S. Provisional Application No.
60/599,916, filed on Aug. 10, 2004, the entire disclosures of which are
herein expressly incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002]In wireless communication systems, mobility between different
geographical areas is provided by allowing mobile stations to handoff
their communications between base stations. The base station from which a
mobile station is handing off from is typically referred to as a serving
base station, and the base station to which the mobile station is to hand
off to is typically referred to as a target base station. Mobile stations
can include cellular telephones, personal digital assistants (PDAs),
computers equipped with wireless transceivers (such as integrated
transceivers or transceivers on PCMCIA cards), pagers, and the like.

[0003]There are two general types of handoff, hard handoff and soft
handoff. Hard handoff is performed in a break-before-make manner, in
which a mobile station ceases all traffic and control channel
communications with the serving base station and then attempts to
establish traffic and control channel communications with the target base
station. This approach is used mostly to preserve air-link and network
resources utilized by mobile stations in the handoff areas, and might be
the preferred mode for high-speed packet data channels. It also
simplifies the handling of data packets at the network infrastructure.
However, hard handoff does not provide full diversity gain associated
with soft handoff, thereby increasing the possibility of temporary
session interruption. Such a temporary interruption would impact Quality
of Service (QoS) for real-time applications, such as voice.

[0004]Using a soft handoff technique, a mobile station will simultaneously
communicate with the serving and target base stations over both base
stations traffic and control channels. This type of handoff provides
diversity, as the mobile station can receive the same information from
both base stations. Due to low chance of session interruption, soft
handoff is typically preferred where real-time applications such as voice
are involved. However, soft handoff requires synchronous resource
allocation and scheduling, and coordinated packet handling on the network
side. This has a direct impact on throughput efficiency and network
complexity. Soft handoff also increases the required backhaul capacity,
especially for high rate traffic channels. Additionally, it is only
feasible when frequency reuse is 1:1 and strict traffic synchronization
is required among all bases stations involved in the soft handoff.
Moreover, soft handoff complicates packet scheduling and automatic
retransmission request (ARQ) messages.

[0005]While many wireless communication technologies provide either soft
or hard handoff, broadband communication technologies including cdma2000,
1xEV-DO, WCDMA, Flarion's Flash OFDM® and IEEE80216e provide both
hard and soft handoff. Specifically, soft handoff is typically employed
for communications that are sensitive to interruptions or errors, while
hard handoff is employed for all other communications.

[0006]Exemplary embodiments of the present invention provide a hybrid
handoff scheme in which control channel communications are handed off
from a serving base station to a target base station, while the mobile
station continues to perform data communications with the serving base
station. Once the handoff of the control channel communications are
completed, the data communications are handed off from the serving base
station to the target base station.

[0007]In accordance with one embodiment of the present invention, the
serving base station informs the mobile station of time periods,
corresponding to frames, during which the mobile station can perform the
signaling with the target base station necessary for handing off the
control channel communications from the serving base station to the
target base station. The serving base station then adjusts its scheduler
in such a way that the serving base station transmits unicast and
multicast messages to the mobile station during time periods other than
those during which the mobile station is performing signaling with the
target base station.

[0008]In accordance with another embodiment of the present invention, a
fast cell switch feedback channel is allocated to the mobile station when
there is a possibility or a need for a handoff. The mobile station uses
the fast cell switch feedback channel to perform the signaling necessary
for handing off the control channel communications from the serving base
station to the target base station. The mobile station then can handoff
the traffic channel communications from the serving base station to the
target base station.

[0009]Other objects, advantages and novel features of the present
invention will become apparent from the following detailed description of
the invention when considered in conjunction with the accompanying
drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a signaling diagram illustrating an exemplary method for
handoff of a mobile station between a serving base station and a target
base station in accordance with one embodiment of the present invention;
and

[0011]FIG. 2 is a signaling diagram illustrating an exemplary method for
handoff of a mobile station between a serving base station and a target
base station in accordance with another embodiment of the present
invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a signaling diagram illustrating an exemplary method for
handoff of a mobile station between a serving base station and a target
base station in accordance with one embodiment of the present invention.
During the time period of Frame 1, the mobile station receives a neighbor
advertisement (NBR-ADV) message broadcast by the serving base station
(BS1) identifying base stations for which the mobile station is to
measure their signals. The mobile station can also receive the downlink
MAP (DL_MAP) and uplink MAP (UP_MAP) during Frame 1. The mobile station
then measures the received carrier-to-interference plus noise ratio
(CINR) of all neighboring base stations advertised in the NBR-ADV message
and transmits a mobile station handoff request (MSSHO-REQ) message during
the time period of Frame 2. The MSSHO-REQ message identifies the measured
base stations and their corresponding CINRs.

[0013]Base stations with CINR's above a predetermined threshold are
considered as "candidate" base stations for handoff. Based on the
received measurements, and other logical considerations, the serving base
station, or the handoff controller entity in the network, selects one of
the bases stations among the top candidate neighbor base stations as the
target base station for handoff. The serving base station, which also
serves as an "anchor" base station during the cell switching process,
notifies the target base station (BS2) that the mobile station will
attempt a handoff and transfer the mobile station's context to the target
base station using, for example, HO Notification and Context Transfer
messages.

[0014]The target base station reserves a ranging channel and basic
connection identifier (CID) for the mobile station, and forwards this
information to the serving base station. The serving base station, during
the time period of Frame 3, transmits a message to the mobile station
instructing it to attempt a handoff to the target base station. The
message includes the target base station's identification, the reserved
ranging channel and the basic CID. The reserved ranging channel can be a
physical channel or a logical channel defined by a code. This message can
be a HO Preparation Order Message, or can be an extension to a base
station handoff response (BSHO-RSP) message. The message also identifies
upcoming time periods, or frames, in which the mobile station is allowed
to be unreachable, herein referred to as "T-Frames". The serving base
station's scheduler is adjusted to prevent scheduling any unicast or
multicast messages for transmission to the mobile station during the
T-Frames. In FIG. 1 time periods, or frames, in which the mobile station
is to communicate with the serving base station are herein referred to as
"S-Frames". Although FIG. 1 illustrates the T-Frames occurring following
every other S-Frame, other schedules of the T-Frames are possible.

[0015]Upon the occurrence of the first T-Frame, i.e., the time period of
Frame 4, the mobile station tunes to the control channel of the target
base station to receive broadcast control messages, such as the frame
control header (FCH), DL_MAP and UL_MAP, and the like. The mobile station
then sends an initial ranging message on the reserved ranging channel to
the target base station. Although not illustrated, during the T-Frames,
the mobile station monitors broadcast information from the target base
station, such as the DL_MAP, UL_MAP, downlink channel descriptor (DCD),
uplink channel descriptor (UCD), or any other signaling information
necessary for handoff. Additionally, the serving base station, as part of
the HO-Preparation message, can identify the current DCD and UCD of the
target base station, if they are different from that of the serving base
station. Otherwise, the mobile station will have to monitor the target
base station's downlink message until it receives the DCD and UCD.
Knowing the DCD and UCD is necessary, as the mobile station cannot start
the initial ranging until it knows the DCD and UCD of the target base
station.

[0016]The mobile station then tunes again to the serving base station
during the next S-Frame, i.e., during the time period of Frame 5, in
order to receive any data from the serving base station. During the next
T-Frame, i.e., the time period of Frame 6, the mobile station again tunes
to the control channel of the target base station to receive the response
to the ranging message (RNG-RSP), as well as a CID allocation. Although
not illustrated, the mobile station and target base station can exchange
other Layer 2 and Layer 3 messages before the mobile station's traffic
channel is switched to the target base station. These messages may be
related to security, system parameter updates, or other information
needed by the mobile station or the base station to carry the session
seamlessly.

[0017]At the next S-Frame, i.e., the time period of Frame 7, the mobile
station tunes to the serving base station and transmits a message
indicating that the mobile station has completed the handoff of the
control channel to the target base station (HO-Completion Indication).
The serving base station forwards an indication of the completed handoff
to the target base station, at which point traffic information is
switched from the serving to target base station. The target base
station, during the time period of Frame 8, transmits a traffic channel
allocation to the mobile station and the target base station becomes the
serving base station for all traffic and control channel communications
for the mobile station.

[0018]When the method of FIG. 1 is performed between base stations that
operate over the same frequency or frequencies, some message exchanges
between the target base station and the mobile station can occur during
the S-Frames. If the mobile station reselects the serving base station
before handoff to the target base station is completed, the serving base
station can terminate the handoff process by notifying the target base
station of the early termination. In this case, the serving base station
will remain as the serving base station, traffic channel frame exchanges
will not be affected, and the mobile station will ignore the T-Frame
allocation.

[0019]FIG. 2 is a signaling diagram illustrating an exemplary method for
handoff of a mobile station between a serving base station and a target
base station in accordance with another embodiment of the present
invention. In accordance with this embodiment of the present invention, a
fast cell switch feedback channel is allocated to the mobile station when
there is a possibility or the need for handoff. This channel allows
timely and efficient notification of cell switching status, before,
during and upon completion of the process.

[0020]As illustrated in FIG. 2, the mobile station receives NBR-ADV and
DL_MAP and UL_MAP messages from the serving base station. The mobile
station then measures the received CINR of all of neighboring base
stations advertised in the broadcast NBR-ADV message. The mobile station
then transmits a MSSHO-REQ message identifying the measured base stations
and their corresponding CINRs.

[0021]All reported neighbor base stations with CINR's above a
predetermined threshold are considered as "candidate" base stations for
handoff. Based on the received measurements, and other logical
considerations, the base station specifies a subset of the candidate
neighbors as recommended base stations for handoff, namely the Active
Set. The serving base station, which also serves as an "anchor" base
station during the cell switching process, allocates a dedicated channel
named "Fast Cell Switch" or FCS to the mobile station. The Active Set and
the FCS channel allocation can be specified through the BSHO-RSP message.
The serving base station also allocates a short, e.g. 3 bit, temporary ID
for each base station in the Active Set. If the system defines an uplink
dedicated control channel for every active user, the FCS may also be
defined as a subchannel or field within uplink dedicated control channel.

[0022]The FCS channel is used by the mobile station to frequently
indicate, e.g. every 5 msec or once a frame, one of the base stations in
the Active Set with highest CINR to be the next target base station. The
mobile station may consider other factors in selecting the target base
station from those in the Active Set.

[0023]Right after FCS allocation the mobile station may specify the
current anchor base station as the target base station for a period of
time until one of the Active Set base station's CINR become stronger than
the serving base station. The system also defines a configurable timer
for the FCS to allow the network to perform necessary signaling with the
target base station, and for the mobile station to establish dual
signaling with the serving base station and target base station in a
make-before-break manner. The mobile station continues exchanging traffic
frames with the serving base station until that timer expires, or when it
successfully completes the ranging with the target base station.

[0024]If the mobile station reselects the serving base station again,
before handoff to the target base station is competed, the serving base
station can stop the cell switching process by notifying the target base
station of the early termination of handoff. Accordingly, the serving
base station does not change and traffic channel frame exchange will not
be impacted. During the handoff process the mobile station continues
measuring the signals from neighbor base stations and updates the
recommended changes to, i.e., additions to and deletions from, the
current Active Set through MSSHO-REQ message.

[0025]New neighbor base stations will be added to Active Set if their
CINR's are above a predetermined threshold, and current Active Set
members may be deleted if their CINR's are below another predetermined
threshold. The FCS will be automatically released by the mobile station
once the serving base station becomes the only member of the Active Set.

[0026]When the mobile station notifies the serving base station of its
desire to switch to the target base station, the serving base station
transmits a message to the target base station informing the target base
station of the handoff in process and to transfer the mobile station's
context information. The serving base station may also send a message
(not illustrated) to the target base station identifying the FCS channel
location, so that the target base station can optionally listen to this
channel during the cell switching process.

[0027]Following the notification, the target base station allocates a
basic CID and a reserved initial ranging CID for the mobile station, and
transmits this information to the serving base station. The serving base
station transmits this information to the mobile station. The mobile
station monitors the broadcast information from target base station, such
as DL/UL_MAP, downlink channel descriptor (DCD), uplink channel
descriptor (UCD), or any other signaling information needed to switch the
traffic channel to target base station. To expedite the process of
switching, the mobile station can start monitoring some of broadcast
messages from target base station as early as the time it sends the
FCS=BS2 message to the serving base station. However, some of information
which requires a specific CID needs to be read once the early target base
station allocation, e.g. basic CID, are forwarded to mobile station from
serving base station.

[0028]Moreover, the mobile station transmits a message to the target base
station on a reserved and contention-free ranging channel allowing the
target base station to perform initial ranging. Once the mobile station
receives the ranging response along with a CID allocation, it can
indicate its handoff completion to the old serving base station by
sending a predefined code word on the FCS channel. The mobile station
then receives a traffic channel (TCH) allocation from the target base
station and the handoff process is completed.

[0029]As the base stations in the Active Set are given a temporary ID,
e.g. 3 bit TEMP_BS_ID, the mobile station can indicate its selected base
station on the FCS channel using this TEMP_BS_ID. One of TEMP_BS_ID's,
e.g. 000, or a separate additional bit can be reserved to indicate the
handoff completion.

[0030]During the context transfer and ranging process with target base
station, the traffic communication is still performed between the mobile
station and serving base station, as long as the FCS timer is not
expired. After the interaction of necessary signaling information, and
performing initial ranging, and the allocation of necessary primary and
traffic CIDs for mobile station in target base station, the mobile
station indicates the handoff completion through the reserved ID on the
FCS. The mobile station then releases that channel. This is the moment
when the traffic communication is switched from the serving base station
to target base station.

[0031]Although the present invention has been described in connection with
a handoff between two different base stations, the present invention can
also be employed for handoffs between two sectors of the same base
station. It will be recognized that intra-base station communication and
channel establishments with the target sector can be performed faster
than the inter-base station processes.

[0032]While the present invention is described in the context of an OFDMA
system, and in IEEE 802.16 specifically, the present invention is equally
applicable to other access technologies and standards.

[0033]The methods described above in connection with FIGS. 1 and 2 can be
used in, for example, high-speed packet data communications in the
control/signaling channels to preserve the link integrity throughout the
handoff process, while the hard handoff can be performed for the traffic
channels. However, if desired, for real-time and low-rate applications,
such as voice, soft handoff can be performed by both traffic and
signaling channels.

[0034]The methods described above in connection with FIGS. 1 and 2 can be
used for both handoff between two base stations with the same frequency,
i.e., intra-frequency handoff, and between base stations of different
frequencies, i.e., inter-frequency handoff. In both cases the described
method allows continuous active communications with the network
throughout the handoff process, even for real-time and constant bit rate
services, possibly at lower data rates, but with negligible handoff
latency.

[0035]The foregoing disclosure has been set forth merely to illustrate the
invention and is not intended to be limiting. Since modifications of the
disclosed embodiments incorporating the spirit and substance of the
invention may occur to persons skilled in the art, the invention should
be construed to include everything within the scope of the appended
claims and equivalents thereof.